In what follows, and by convention, the circumferential, axial and radial directions respectively denote a direction tangential to the tread surface of the tire in the direction of rotation of the tire, a direction parallel to the axis of rotation of the tire, and a direction perpendicular to the axis of rotation of the tire. “Radially on the inside or respectively radially on the outside” means “respectively closer to or further away from the axis of rotation of the tire”. “Axially inside and respectively axially outside” mean “respectively closer and further away from the equatorial plane of the tire”, the equatorial plane of the tire being the plane that passes through the middle of the tread surface of the tire and perpendicular to the axis of rotation of the tire. Angles with respect to the circumferential direction which are mentioned hereinbelow are indicated in terms of their absolute value disregarding their orientation.
A tire comprises a tread which is intended to come into contact with the ground via the tread surface and is connected by two sidewalls to two beads that provide the mechanical connection between the tire and the rim on which it is mounted.
A radial tire also comprises a reinforcement comprising a crown reinforcement, radially on the inside of the tread, and a carcass reinforcement, usually radially on the inside of the crown reinforcement.
The crown reinforcement of a radial tire for a motorbike generally comprises at least one crown layer made up of reinforcers coated in a polymer material of the elastomer type. The reinforcers are usually made up of a textile material such as aramid, but may also be made of metals. Various crown reinforcement architectures have been proposed by those skilled in the art according to whether the tire is intended to be fitted at the front or fitted at the rear of the motorbike. For fitting at the front, the crown reinforcement usually comprises at least two working crown layers, comprising reinforcers that are substantially mutually parallel within each layer and crossed from layer to the next, forming, with the circumferential direction, angles generally of between 15° and 35°. The working crown layers may be associated with at least one circumferential crown layer, comprising circumferential reinforcers, i.e. forming, with the circumferential direction, a substantially zero angle, at most equal to 5°. This circumferential crown layer is generally obtained by helical winding of a strip of at least one elastomer-coated reinforcing element. For fitting at the rear, the crown reinforcement is usually made up of a circumferential crown layer.
The carcass reinforcement of a radial tire for a motorbike generally comprises at least one carcass layer made up of reinforcers usually made of a textile material and coated in a polymeric material of the elastomer type. A carcass layer may have a turn-up or no turn-up.
A carcass layer is said to have a turn-up when it comprises a main part, connecting the two beads together and turned up, within each bead, from the inside of the tire outwards around a bead wire to form a turn-up with a free end. The bead wire in a circumferential reinforcement element, usually made of metal, and coated in a material generally elastomeric or textile. In the case of a carcass layer with a turn-up the turn-up, within each bead, anchors the carcass layer comprising it to the bead wire. The portion of bead wire in contact with the turned-up carcass layer contributes, particularly on inflation, to the ability of the turned-up carcass layer to absorb tension forces through coupling. This contribution to the absorption of tension forces is dependent on the torsional rigidity of the bead wire and on the geometry of the turn-up. If the bead wire has high torsional rigidity, the tension forces on inflation are essentially absorbed by the bead wire, with the turn-up playing a secondary role. If the bead wire has a lower torsional rigidity then the tension forces are absorbed both by coupling with the bead wire and by shearing between the turn-up and the materials adjacent to it, entailing a turn-up that is sufficiently long, namely the end of which is sufficiently radially distant from the radially innermost point of the bead wire. A turn-up is said to be long when the radial distance between its end and the radially innermost point of the bead wire is at least equal to 0.3 times the design section height of the tire as defined by the European Tire and Rim Technical Organisation or ETRTO standards.
A carcass layer has no turn-up when it is made up only of a main part connecting the two beads together, without being turned up around a bead wire. In the case of a carcass layer with no turn-up, each of the two end portions of the said carcass layer with no turn-up may be coupled either with the turn-up of at least one carcass layer that does have a turn-up, or with the main part of at least one carcass layer that does have a turn-up. Coupling means an area of overlaps between the carcass layer with no turn-up and a carcass layer with a turn-up, that allows tension forces to be absorbed in shear.
The reinforcers of the main part of a carcass layer with or without a turn-up are substantially parallel to one another and form, with the circumferential direction, an angle of between 65° and 90°.
A first known radial tire design for fitting at the rear of a motorbike comprises a carcass reinforcement made up of a carcass layer with a turn-up and a carcass layer without a turn-up, radially on the inside of a crown reinforcement consisting of a circumferential crown layer. The carcass layer without a turn-up is axially on the outside of the main part of the carcass layer with a turn-up but axially on the inside of the turn-up of the carcass layer with a turn-up. The turned-up end of the turned-up carcass is radially on the inside of the end of the circumferential crown layer, which means there is no overlap between the turn-up of the carcass layer with a turn-up and the circumferential crown layer. The respective reinforcers of the carcass layer with a turn-up and of the carcass layer without a turn-up form, with the circumferential direction, angles that are substantially equal in terms of absolute value, of opposite sign and comprised between 65° and 85°. Such a design, because of the triangulation effect resulting from the crossing of the reinforcers from one carcass layer to another, particularly in the side walls, guarantees the motorbike satisfactory stability when following a curved path. The camber angle of the rear tire, between the equatorial plane of the tire and the plane perpendicular to the ground and tangential to the path, is usually at least equal to 30° in a curved path.
A second known design of radial tires for mounting at the rear of a motorbike comprises a carcass reinforcement consisting of a single carcass layer with a turn-up, radially on the inside of a crown reinforcement consisting of a circumferential crown layer. The end of the turn-up of the turned-up carcass is radially on the inside of the end of the circumferential crown layer, which means there is no overlap between the turn-up of the carcass layer with a turn-up and the circumferential crown layer. The reinforcers of the carcass layer with a turn-up form, with the circumferential direction, an angle close to 90° and in practice comprised between 80° and 90°. Such a design, because of the coupling between the radial carcass layer and the circumferential crown layer in the crown region of the tire, guarantees the motorbike satisfactory stability when following a path in a straight line. When a path follows a straight line, a motorbike is usually travelling at high speed, at least equal to 150 km/h.